sfxge: implement SIOCGI2C to read infromation from phy modules

[arybchik]

Summary

The IOCTL is used by 'ifconfig -v' to show SFP+/QSFP+ information
including inventory information and dianostics (temperature, light
levels, voltage etc).

Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days

Test Plan

Build tested. Driver loaded/unloaded.
ifconfig -v check on 5xxx/6xxx with SFP+ tranceiver and 7xxx NICs with
SFP+ and QSFP+.

sfxge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500

options=ec07ab<RXCSUM,TXCSUM,VLAN_MTU,JUMBO_MTU,VLAN_HWCSUM,TSO4,TSO6,LRO,VLAN_HWTSO,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
ether 00:0f:53:21:17:20
inet 192.168.0.1 netmask 0xffffff00 broadcast 192.168.0.255 
nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
media: Ethernet autoselect (10Gbase-SR <full-duplex,rxpause,txpause>)
status: active
plugged: SFP/SFP+/SFP28 10G Base-SR (LC)
vendor: Intel Corp PN: AFBR-703SDZ-IN2 SN: AD1148A02NS DATE: 2011-12-14
module temperature: 54.60 C Voltage: 3.31 Volts
RX: 0.20 mW (-6.98 dBm) TX: 0.55 mW (-2.55 dBm)

[melifaro]

Great news, thanks for implementing that!